Xylenes are valuable industrial chemicals. Xylenes are derived primarily from aromatic naphthas such as petroleum reformates and pyrolysis gasolines. Petroleum reformates result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from steam cracking of hydrocarbons to manufacture ethylene, propylene, and the like.
Generally, regardless of the aromatic naphtha source, it has been the practice to subject the liquid hydrocarbon to extraction with a solvent highly selective for aromatics to obtain an aromatic mixture of the benzene and alkylated benzenes present in the aromatic naphtha. The resulting extract can then be distilled to separate benzene, toluene and C.sub.8 aromatics from higher boiling compounds in the extract. Benzene and toluene are recovered in high purity; however, the C.sub.8 fraction, containing valuable para-xylene, is a mixture of three xylene isomers with ethylbenzene. These mixtures will also contain C.sub.8 -C.sub.9 paraffins, the amount of which is determined both by the source of the naphtha as well as the efficiency of the solvent extraction.
As commercial use of para- and ortho-xylene has increased, there has been interest in isomerizing the xylene isomers toward an equilibrium mix and thus increasing yields of the desired xylenes. Of the xylene isomers, i.e., ortho-, meta- and para-xylene, meta-xylene is the least desired product, while ortho- and para-xylene are the most desired products. Para-xylene is of particular value as it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of polyesters, and of synthetic fibers such as "Dacron".
In practice, isomerization processes are operations used in conjunction with xylene separation processes. A virgin C.sub.8 aromatics mixture is fed to such a combined process system, along with undesired isomers emerging from the product separation steps. The feed is charged to the isomerizing unit and the effluent isomerizate C.sub.8 aromatics are sent to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance. The objective in the isomerization reactor is to bring the charge as near to the equilibrium concentration as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation. The thermodynamic equilibrium varies slightly with temperature.
The rate of ethylbenzene conversion in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure can have a very significant effect on ethylbenzene conversion. Products formed from ethylbenzene include C.sub.8 naphthenes, benzene from hydrocracking ethylbenzene and C.sub.9.sup.+ aromatics from disproportionation, as well as total loss to other than C.sub.8 molecular weight components, such as C.sub.5 and lighter hydrocarbon byproducts.
By comparison, the three xylenes isomerize much more selectively than does ethylbenzene. However, the xylenes do exhibit different rates of isomerization and hence, with different feed composition situations, the rates of approach to equilibrium vary considerably. Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 and heavier aromatics and C.sub.5 and lighter hydrocracking products.
Because of the deleterious effects of ethylbenzene build up in the loop manufacture of the xylenes and because of the great expense of removing it from mixed C.sub.8 aromatics, a process which would result in ethylbenzene conversion at a rate approaching that of xylene isomerization would be desirable provided xylene losses can be maintained at a reasonable level. Progress toward such a goal was heralded by U.S. Pat. No. 4,163,028 which describes a catalyst and its use in an isomerization process conducted at 800.degree. to 1000.degree. F.; the catalyst described in 4,163,028 comprises a zeolite having a constraint index of about 1 to 12 and having a silica/alumina ratio of at least 500. An improved catalyst for that aforementioned goal was later described in U.S. Pat. No. 4,312,790.
However, even the use of catalysts of U.S. Pat. Nos. 4,163,028 and 4,312,790 can result in unacceptively high xylene losses. For example, we have determined that catalysts of U.S. Pat. Nos. 4,163,028 and 4,312,790 give unacceptably high xylene losses when xylene isomerization feeds containing greater than 15% ethylbenzene are processed under conditions which give greater than about 50% conversion of ethylbenzene per pass; that situation can be further aggravated if the isomerization feed contains paraffins. The catalyst system of the present invention does not suffer from these limitations.